Atherosclerosis and its complications remain leading causes of death and disability in the United States. Although promoting the regression of atherosclerosis has emerged as a promising treatment strategy for atherosclerosis, the molecular and cellular mechanisms that orchestrate this clinically relevant process are poorly understood. Moreover, the mechanisms by which the apolipoprotein 4 (apoE4) allele predisposes to symptomatic forms of atherosclerosis and whether it can impair its regression following plasma lipid lowering, remains largely unknown. To fill these gaps of knowledge, we propose a series of experiments with genetically engineered murine models of apoE4, conditional apoE expression, reversible hyperlipidemia, and atherosclerosis regression developed in our laboratory. Hypomorphic ApoeR61h/h mice express reduced levels of an apoE4-like form of murine apoE (Arg-61 apoE) that reproduces a unique biophysical feature of human apoE4 called "domain interaction". Our previous studies have shown that domain interaction in Arg- 61 apoE is sufficient to reproduce the VLDL-binding preference of human apoE4 in hyperlipidemic mouse plasma. Moreover, our studies of conditional expression of apoE in ApoeR61h/h mice has recently established the existence of a role for apoE in promoting the regression of atherosclerosis beyond lowering plasma cholesterol levels. Experiments detailed in this proposal will make use of ApoeR61h/h mice, as well as new variants of this model, to investigate mechanisms by which apoE accumulation in plasma and macrophage-derived apoE in lesions regulate the onset and progression of atherosclerosis in the setting of hyperlipidemia. We will also test the hypothesis that apoE4 domain interaction in Arg-61 apoE will accelerate the progression of atherosclerosis and impair its regression following sustained lipid lowering. Lastly, we will test the hypothesis that apoE expression levels influence the migration of monocytes into established lesions, and the egress of macrophages from lesions during the regression of atherosclerosis in an isoform-specific manner. Our long term goal is to clarify the molecular and cellular mechanisms by which apoE isoforms regulate the progression and regression of atherosclerosis. A better understanding of these mechanisms would help usher in a new generation of molecular therapies to fight atherosclerosis, particularly among apoE4 carriers who represent 20% of the global population.